Letterman then notes, "She kicked you in the head and she didn't mention a thing about it."

Turns out, Josh suffered a concussion from Jennifer's kick - and it wasn't even when they were filming!

Hutcherson explains, "We were just goofing around, and like doing some shadow boxing for fun... as one does," adding, "She throws this beautiful kick, but it landed right on my temple and literally knocked me to the ground and I was down."

Jennifer was upset and crying and he was telling her it was going to be okay - little did he know that the injury was more severe.

He felt "dizzy and out of it," but soon realized something was wrong: "The next day somebody asked me for my address and I started telling them my address from when I was nine years old. That's when I was like, This is not right."

Josh suffered a mild concussion and he said "Jennifer felt awful." Aw.

Letterman asks why were they doing it? It seems that when you're making a movie, there's a lot of downtime on set and actors need to entertain themselves.

Kick boxing is one way to do it!

The stars of 'The Hunger Games' were also in top shape, as Josh shares in this UsMagazine.com interview, noting that he had to "put on a lot of weight" and work out with "epic trainer" Logan Hood who really put him through his paces.

Josh explains, "He was this ex-Navy Seal guy who just kicks ass" who worked him so hard, he was in "a constant state of soreness."

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